Size reduction and high-density mounting of electronic components are a fast growing trend in the recent electronic parts assembly technology. With this trend, a demand is increasing for higher reliability and performance of soldering materials in correspondence to narrow-pitched electronic parts. Meanwhile, the concern for environmental protection is increasing and there is a movement to legally regulate disposal of industrial wastes including electronic circuit boards.
Hereinafter, a conventional soldering material will be described with reference to drawings. FIG. 2 shows an alloy composition of a conventional soldering material and its metallic structure in a bonding interface between a copper land and a solder. In FIG. 2, reference numeral 1 indicates an .alpha.-solid-solution Sn layer. 2 indicates a .beta.-solid-solution Pb layer. 3 indicates an intermetallic compound composed of Cu.sub.3 Sn. 4 also indicates an intermetallic compound composed of Cu.sub.6 Sn.sub.5. Indicated by 5 is a Cu land.
The aforesaid conventional solder is an eutectic alloy composed of Sn and Pb, having an eutectic point of 183.degree. C., in which Sn accounts for 63 weight % and Pb 37 weight %. The alloy contains the .alpha. solid solution 1 and the .beta. solid solution 2 in lamellar states. Further, the intermetallic compounds 3 and 4 are formed in the bonding interface between the copper land and the solder.
From the environmental protection view point, however, such a movement as to restrict the use of lead which is a toxic substance in a Sn--Pb alloy solder is rapidly spreading all over the world. The conventional solder material in which the alloy has a lamellar structure poses a problem that the alloy components swell at a high temperature and the solder is stressed thereby, whereby the interfaces between the alloy components are caused to slide, resulting in soldering cracks. There is another problem that, during soldering or at high temperatures, a rigid and brittle two-layered intermetallic compound is produced in the bonding interface between the solder and the copper land, whereby cracks are caused to take place in the interface.